Time of flight ion analysis with a pulsed ion source employing ion-molecule reactions

ABSTRACT

Apparatus and methods for sorting and detecting ions in a drift cell, the electric fields applied to the cell being controlled at appropriate times to minimize dispersion of bunched ions produced by a pulsed source. Bunched product ions produced by ion-molecule reactions are analyzed in accordance with their velocity in a drift field.

United States Patent Inventor Appl. No.

Filed Patented Assignee Martin J. Cohen West Palm Beach, Fla. 812.285

Apr. 1, 1969 July I3, 1971 Franklin GMO Corporation West Palm Beach,Fla.

TIME OF FLIGHT ION ANALYSIS WITH A PULSED ION SOURCE EMPLOYINGION-MOLECULE REACTIONS I9 Claims, 2 Drawing Figs.

U.S. Cl 250/413 TF, 250/419 G, 250/42.9 SB

Int. Cl ..H0lj 39/34, BOId 59/44, HOlj 37/08 Primary Examiner lames W.Lawrence Assistant Examiner-C. E. Church Attorney- Raphael SemmesABSTRACT: Apparatus and methods for sorting and detect ing ions in adrift cell, the electric fields applied to the cell being controlled atappropriate times to minimize dispersion of bunched ions produced by apulsed source. Bunched product ions produced by ion-molecule reactionsare analyzed in accordance with their velocity in a drift field.

PULSED SOURCE CURRENT TIME 1 ION PULLOUT (b) V REACTION DRIFT 50m sec50m sec TIME 5m see A ION PULLOUT REACTION DRIFT K n r1 r1 in LJ LJ LiLi L! TIME? PATENT-ED JUL I 3 IQTI 2 IO I --A K PULSED ION SOURCE STATICAND DYNAMIC POTENTIAL SUPP L Y INVENTOR MARTIN J. COHEN TIMEOF FLIGHTION ANALYSIS WITH A PULSED ION SOURCE EMPLOYING ION-MOLECULE REACTIONSBACKGROUND OF THE INVENTION This invention relates to apparatus andmethods of ion classification and more particularly is concerned withenhancing the resolution of ion measurements performed in a drift cell.

The copending application of Martin J. Cohen, David I. Carroll, Roger F.Wernlund, and Wallace D. Kilpatrick Ser. No. 777,964, filed Oct. 23,i968, and entitled Apparatus and Methods for Separating, Concentrating,Detecting, and Measuring Trace Gases," discloses "Plasma Chromatography"systems involving the formation reactant ions and the reaction of theseions with molecules of trace substances to form product ions, which maybe concentrated, separated, de-

tected, .and measured by virtue of the velocity or mobility of the ionsin an electric field. The production and analysis of ions takes place ina chamber, the length ofthe meanfree path of the ions being very muchless than the dimensions of the chamber under operating pressureconditions, such as atmospheric. The reactant ions may be produced bysubjecting the molecules of a suitable host gas, such as air, toionizing radiation, for example. The reactant ions are subjected to anelectric drift field, causing them to migrate in a predetermineddirection through a reaction space into which the sample or trace gas isintroduced. The resultant collisions between the reactant ions and thetrace gas molecules produce product ions of the trace gas in muchgreater numbers than can be produced by mere electron attachment, forexample, to the trace gas molecules. The product ions are also subjectedtothe electric drift field and may be sorted in accordance with theirvelocity or mobility. A specific system of the copending applicationemploys a pair of successively arranged ion shutter grids or gates forsegregating the ion species in accordance with their drift time. Theopening, of the first gate. is timed to pass a group of ions, which maycomprise unreacted primary or reactant ions'as well as secondary orproduct ions, and the opening of the second gate is timed to pass aportion of the group to an ion detection means. Only those ions passedduring the short period when the first shutter grid is open form themixed ion bunch which is analyzed by ion drift time in the ion driftanalyzer space. Using a radioactive or other continuous ionizing source,99 percent or more of the available ions are not utilized.

The copending application of Martin J. Cohen entitled Apparatus andMethods for Improving the Sensitivity of Ion Detection and Measurement,filed concurrently herewith, discloses apparatus and methods for ionmeasurements,.employing continuous ion sources, in which a much largerpercentage of the available ions is passed to the ion analysis region..The invention of that application is capable of producing signalimprovements of the order of to 100 times and involves the. rapidwithdrawal of the continuously formed ions from the ion-moleculereaction region and the bunching of such ions before drift-timeanalysis.

BRIEF DESCRIPTION OF THE INVENTION The present invention is concernedwith apparatus and methods for ion measurements in which a pulsedsource,

rather than a continuous source, is employed to produce a bunch or pulseof product ions by ion-molecule reactions with a bunch or pulse ofreactant ions, and in which dispersion of the product ions is minimized.It is accordingly a principal object of the invention to provideapparatus and methods of this type which permit measurements withgreatly enhanced resolution.

Briefly stated, the concept underlying the present invention involvesrapid pullout of primary or reactant ions from the region of a pulsedion source, the holding up of the pulled-out primary ions at a regionadjacent to the region of the source, in order to permit ion-moleculereactions to proceed for a predetermined period andin order toforrn abunch-of product ions, and thereafter the analysis of the product ionbunch in accordance with the velocity of the various species comprisingthe bunch, and the detection and measurement of the ion species ofinter.

BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION OF THE INVENTIONThe drift cell 10 which may be employed in the present invention is ofthe type set forth in the :said copending applica-' tion Ser. No.777,964, and comprises an envelope 12 enclosing a series of electrodes,which may be of parallel plane geometry, for example. Principalelectrodes K and A may be arranged adjacent to opposite ends of theenvelope. When the apparatus is used to detect negative ions, electrodeK will be a cathode and electrode A an anode. When the apparatus is usedto detect positive ions, the polarities will be reversed. Electrode K orthe region of the envelope near this electrode is provided with a pulsedionizing source, such as a pulsed corona source, apulsed spark gap, apulsed RF source, or a pulsed ultraviolet source, all of which are wellknown in the art. Electrode A may be a collector plate constituting anoutput electrode and may be connected to an electrometer (not shown),such as a Cary Instruments Model 401 (vibrating reed) type withsensitivity of 10' amps at a time constant of 300 milliseconds. Thedrift cell employs a grid 14 adjacent to electrode A, which may be adual grid ion gate comprising two sets of interdigitated parallel wires,the sets normally being maintained at equal and opposite potentialsrelative to a grid average potential established by the static anddynamic potential supply 16. In this condition, the grid is closed tothe passage of electrically charged particles, but when, atpredetermined times, all of the elements of the grid are driven to thesame potential (the grid average potential) by the use of appropriategrid drive circuits in the supply 16, the shutter grid opens. Such adual grid and its drive circuits are well known in the prior art and donot per se constitute the present invention. Alternatively, and forsimplicity, a passive grid of simple parallel wire form may be employed,in which event the output electrode A may be connected to the input of afast amplifier and a boxcar integrator, for example. Both types ofdetection and readout circuits are described in the copendingapplication of Roger F. Wernlund, entitled Apparatus and Methods forDetecting, Indicating and Recording Signals Produced by Ionized TraceSubstances in a Gaseous Sample," Ser. No. 798,399, filed Feb. 11, 1969.

The static and dynamic potential supply 16 provides static and dynamicpotentials appropriate to the various electrodes of the drift cell 10,which may also include a series of guard rings 18 spaced along theenvelope for maintaining the uniformity of the drift field in thedifferent regions of the envelope. Inlet 20 isprovide'd for introducinggaseous samples to the envelope, and outlet 22 permits the withdrawal ofgas from the envelope. The pressure in the envelope 12 between theelectrodes K and A is substantially uniform, since the. space betweenthe electrodes is unrestricted, and is such that the length of the meanfree path of the ions is very much less than the dimensions of theenvelope, atmospheric pressure being.

molecules of a host gas, such as air introduced through inlet 20, toelectrons generated by the pulsed source. The primary ions, which may bepositive oxygen ions, for example, are indicated in curve (a) of FIG. 2at time t=0.00. The problem is now to provide sufficient time forion-molecule reactions between the primary ions and the molecules of thetrace substances to be detected and yet to preserve the bunch whileavoiding recombination losses at the source. In accordance with theinvention, this is achieved by the use of a properly times sequence ofvoltage pulses applied across the cell.

First, a voltage pulse (eg. 3,000 volts) is applied across the cell toseparate the desired sign of charged particles from the electrode K. Forpositive ions, the ion pullout pulse is positive, as shown by thepotential V,,-, curve (b) of FIG. 2, that is, the electrode K is drivenpositive relative to electrode A, which may be at ground potential. Themagnitude and duration is such as to move the positive ions in 5milliseconds, for exam ple, to a position 1 cm., for example, fromelectrode K, as indicated by the shaded block in FIG. 1. Then the ionpullout pulse voltage (and, preferably, the grid average voltage) isreduced to zero, as shown by curve (b). At this time, the bunch ofreactant ions, which may extend over a distance of 0.1 cm., for example,will remain in position in the absence of a field at the region of theions. The pulse cloud tends to widen somewhat by diffusion and spacecharge repulsion.

Ion-molecule reactions between the primary or reactant ions and themolecules of trace substances in the sample proceed for 50 milliseconds,for example, the objective being to form a bunch of product ions attheposition of the reactant ions. The 50-millisecond reaction interval isindicated in curve (b). Since the number of reactant ions is many ordersof magnitude less than the number of trace molecules, depletion of thetrace material is not a problem.

After the 50-m'illisecond reaction interval, the potential across thecell is raised to the normal ion-drift potential (e.g. 3,000 volts), andwith a drift distance of, say, l0c., 50-milliseconds drift time issufficient to accomplish the desired ion separation. The drift intervalis indicated in curve (b) also. It is desirable to maintain the ionpulse width as small as possible, say 0.2 cm. Then the resolution isroughly 50in space dimensions. During the drift interval, the averagepotential of grid 14 is also raised (say to 2,700 volts).

If grid 14 is a dual grid of the type set forth previously, this gridwill be opened at some time during the drift interval to pass selectedion species to the output electrode A. By scanning the time of openingof grid 14 relative to the commencement of the drift interval, acomplete spectrum of the ion population within the analysis region maybe produced for recording as a curve of output current versus time.Peaks in this curve represent the different primary and secondary ionspecies. If grid 14 is a simple, nonshutter grid, it will serve toshield the output electrode A and associated circuits fromiondisplacement currents and will always be open.

Instead of reducing the drift voltage to zero during the reactioninterval, an oscillating potential may be applied across the cell tojiggle the ions back and forth for 0.1 cm. distance, for example, asindicated by the pulsating drift potential in the 50- millisecondreaction interval shown in curve (c) of FIG. 2. For example, a 200 Hzoscillating field of 15 volts per centimeter may be used.

The static and dynamic potential supply 16 may be of conventionaldesign. For example, nominal electrode potentials may be established,where required, by a bleeder resistor string from a DC supply ofappropriate polarity. The required pulses may be generated withsolid-state circuitry utilizing a transformer to carry an RF frequencywhich may be rectified at the electrode connections. Pulse voltages canbe clipped with Zener or corona tube regulation. Grid opening andclosing pulses for the shutter grid 14, if such a grid is employed, areof course superimposed upon the corresponding grid elements.Alternatively, the electrode potentials may be supplied by a bleederstring having taps for explicitly providing all of the voltages needed.A relay or motor-driven switch may then be employed to connect theelectrodes to the appropriate points of the bleeder in sequence.

A special nonreactant or inert gas, such as nitrogen, may be introducedto the analysis region, as set forth in the copending application ofDavid I. Carroll, Martin J. Cohen and Roger F. Wernlund, Ser. No.780,851, filed Dec. 3, 1968, and entitled Apparatus and Methods forSeparating, Detecting, and Measuring Trace Gases with EnhancedResolution. Since the sample volume in the drift cell is defined by theregion where the reactant ions rest, the volume for nonreactant gas isdefined accordingly. Similarly, when the apparatus of the invention isused as a gas chromatography detector (the gas chromatograph effluentbeing introduced to the envelope 12 in addition to a suitable reactantgas), an appropriate small gas chromatography sample volume is definedby the reactant ion rest region.

While preferred embodiments of'the invention have been shown anddescribed, it will be apparent to those skilled in the art that changescan be made in these embodiments without departing from the principlesand spirit of the invention, the scope of which is defined in theappended claims.

The invention I claim is:

l. A method of ion analysis in a drift field which comprises forming abunch of reactant ions, adjusting the drift field to hold said bunch ofions in a predetermined region while reacting said ions with moleculesof a sample to form a bunch of product ions, increasing the drift fieldto cause ions present after reaction to separate in accordance with thevelocity of the ions in the drift field, the foregoing steps beingperformed under substantially the same gas pressure, detecting at leasta portion of the se arated ions.

2. A method in accordance with claim 1, wherein the bunch of reactantions is formed during a first interval of time and is reacted during asecond interval of time which is longer than said first interval.

3. A method in accordance with claim 1, wherein the reactant ions aremoved to said predetermined region by the application ofa drift fieldthereto.

4. A method in accordance with claim 1, wherein the reac tant ions aremaintained substantially stationary during the reacting.

5. A method in accordance with claim 1, wherein the reactant ions arejiggled during the reacting.

6. A method in accordance with claim 1, wherein the pressure issubstantially atmospheric.

7. A method of improving the resolution of ion analysis in a drift cell,which comprises forming a bunch of reactant ions at a first region ofthe cell, moving said ions from said first region to a second region ofthe cell as a bunch then maintaining said ions substantially at saidsecond region while reacting said ions with sample gas molecules to forma bunch of product ions, thereafter moving the bunch of product ions,and any reactant ions remaining after reaction, through an analysisregion and causing them to separate in accordance with their driftvelocity, detecting at least a portion of the separated ions.

8. A method in accordance with claim 7, wherein a drift field is appliedacross said drift cell during reactant ion formation and movement tosaid second region, the field is then substantially reduced across saidcell during the reacting, and the field is then increased during theseparating.

9. A method in accordance with claim 7, wherein said bunch of reactantions is formed by actuating a pulsed ionizing source.

10. Apparatus for ion measurements, comprising an envelope, a pair ofelectrodes spaced apart in said envelope, a pulsed ionizing sourceadjacent to one of said electrodes, means for introducing a gaseoussample into said envelope whereby a bunch of reactant ions is formedadjacent said source from reactant molecules of said sample when saidsource is actuated, means for applying a drift field between saidelectrodes during a first interval of time to draw said ions away fromsaid source, means for substantially reducing said drift field during asecond interval of time for maintaining said bunch of ions at apredetermined region to permit said ions to react with'tracemoleculesfrorn said sample to form a bunch of productions, means forapplying a drift field between said electrodes during a third intervalof time for causing the product ions and any unreacted reactant ions tobe separated in accordance with their drift velocity, and means fordetectin g at least a portion of the separated ions.

11. Apparatus in accordance with claim 10, wherein the space betweensaid electrodes is substantially unrestricted and has a substantiallyuniform gas'pressure distribution.

12. Apparatus in accordance with claim 10, wherein said detection meanscomprises the other of said electrodes, there being a grid between saidelectrodes adjacent to said other electrode.

13. Apparatus in accordance with claim 12. wherein said grid has meansfor applying a potential thereto for shielding said other electrode fromion displacement currents in said cell.

14. Apparatus in accordance with claim 12, wherein said grid is an iongate and has means for opening the gate at a a predetermined time.

15. Apparatus in accordance with claim 10, further comprising an iongate between said electrodes adjacent to the other electrode, and meansfor opening said gate at a predetermined time for passing at least aportion of the separated ions to said other electrode.

16. Apparatus in accordance with claim 10, wherein said means forreducing said drift field comprises means for producing an oscillatingdrift field between said electrodes.

17. Apparatus in accordance with claim 10, wherein said second and thirdintervals of time are substantially longer than said first interval oftime.

l8. A method in accordance with claim 7. wherein the recited steps areperformed under substantially the same gas pressure.

[9. A method in accordance with claim 18, wherein the pressure is suchthat the length of the means free path of said ions is very much lessthan the dimensions of the cell.

1. A method of ion analysis in a drift field which comprises forming abunch of reactant ions, adjusting the drift field to hold said bunch ofions in a predetermined region while reacting said ions with moleculesof a sample to form a bunch of product ions, increasing the drift fieldto cause ions present after reaction to separate in accordance with thevelocity of the ions in the drift field, the foregoing steps beingperformed under substantially the same gas pressure, detecting at leasta portion of the separated ions.
 2. A method in accordance with claim 1,wherein the bunch of reactant ions is formed during a first interval oftime and is reacted during a second interval of time which is longerthan said first interval.
 3. A method in accordance with claim 1,wherein the reactant ions are moved to said predetermined region by theapplication of a drift field thereto.
 4. A method in accordance withclaim 1, wherein the reactant ions are maintained substantiallystationary during the reacting.
 5. A method in accordance with claim 1,wherein the reactant ions are jiggled durIng the reacting.
 6. A methodin accordance with claim 1, wherein the pressure is substantiallyatmospheric.
 7. A method of improving the resolution of ion analysis ina drift cell, which comprises forming a bunch of reactant ions at afirst region of the cell, moving said ions from said first region to asecond region of the cell as a bunch then maintaining said ionssubstantially at said second region while reacting said ions with samplegas molecules to form a bunch of product ions, thereafter moving thebunch of product ions, and any reactant ions remaining after reaction,through an analysis region and causing them to separate in accordancewith their drift velocity, detecting at least a portion of the separatedions.
 8. A method in accordance with claim 7, wherein a drift field isapplied across said drift cell during reactant ion formation andmovement to said second region, the field is then substantially reducedacross said cell during the reacting, and the field is then increasedduring the separating.
 9. A method in accordance with claim 7, whereinsaid bunch of reactant ions is formed by actuating a pulsed ionizingsource.
 10. Apparatus for ion measurements, comprising an envelope, apair of electrodes spaced apart in said envelope, a pulsed ionizingsource adjacent to one of said electrodes, means for introducing agaseous sample into said envelope whereby a bunch of reactant ions isformed adjacent said source from reactant molecules of said sample whensaid source is actuated, means for applying a drift field between saidelectrodes during a first interval of time to draw said ions away fromsaid source, means for substantially reducing said drift field during asecond interval of time for maintaining said bunch of ions at apredetermined region to permit said ions to react with trace moleculesfrom said sample to form a bunch of product ions, means for applying adrift field between said electrodes during a third interval of time forcausing the product ions and any unreacted reactant ions to be separatedin accordance with their drift velocity, and means for detecting atleast a portion of the separated ions.
 11. Apparatus in accordance withclaim 10, wherein the space between said electrodes is substantiallyunrestricted and has a substantially uniform gas pressure distribution.12. Apparatus in accordance with claim 10, wherein said detection meanscomprises the other of said electrodes, there being a grid between saidelectrodes adjacent to said other electrode.
 13. Apparatus in accordancewith claim 12, wherein said grid has means for applying a potentialthereto for shielding said other electrode from ion displacementcurrents in said cell.
 14. Apparatus in accordance with claim 12,wherein said grid is an ion gate and has means for opening the gate at apredetermined time.
 15. Apparatus in accordance with claim 10, furthercomprising an ion gate between said electrodes adjacent to the otherelectrode, and means for opening said gate at a predetermined time forpassing at least a portion of the separated ions to said otherelectrode.
 16. Apparatus in accordance with claim 10, wherein said meansfor reducing said drift field comprises means for producing anoscillating drift field between said electrodes.
 17. Apparatus inaccordance with claim 10, wherein said second and third intervals oftime are substantially longer than said first interval of time.
 18. Amethod in accordance with claim 7, wherein the recited steps areperformed under substantially the same gas pressure.
 19. A method inaccordance with claim 18, wherein the pressure is such that the lengthof the means free path of said ions is very much less than thedimensions of the cell.